Over the decades, wireless communication systems have become more and more technologically advanced, with performance increasing in terms of smaller size, operation at higher frequencies and the accompanying increase in bandwidth, lower power consumption for a given power output, and robustness, among other factors. The trend toward better communication systems puts ever-greater demands on the manufacturers of these systems.
Today, the demands of satellite, military, and other cutting-edge digital communication systems are being met with microwave technology, which typically operates at frequencies from approximately 500 MHz to approximately 60 GHz or higher. Many of these systems use bandpass filters to reduce noise or other unwanted frequencies that may be present in microwave signals.
One popular filter used for narrow bandwidth applications is the SAW (surface acoustic wave) filter, which is typically used for applications involving frequencies from the VHF through L bands. SAW filters have the disadvantage of being electrostatic sensitive, and at higher frequencies they have the disadvantage of being lossy. For example, due to coupling inefficiencies, resistive losses, and impedance mismatches, SAW filters become prohibitively lossy at frequencies above approximately 0.8 GHz. At even higher frequencies, such as a few GHz, SAW filters are bounded by sub-micron electrode geometries.
Another typical implementation of bandpass filters uses evanescent mode waveguides. An evanescent mode waveguide may have a conducting tube having an arbitrary cross-sectional shape and having at least one resonator. The dimensions of the cross-section are chosen to allow wave propagation at the operating frequency of interest while causing other frequencies to rapidly decay. A sectional length of an evanescent mode waveguide can be represented as a pi or tee section of inductors whose values are functions of section length, dielectric constant, and guide cross section. A resonant post may be inserted in such a way that it penetrates the broad wall of the evanescent mode waveguide, thereby forming a shunt capacitive element between opposite conducting walls of the guide. The resulting combination of shunt inductance and shunt capacitance forms a resonance. By placing multiple resonator posts spaced at varying distances along a waveguide, multiple resonances are introduced resulting in a wide variety of bandpass functions. The resulting filter is a microwave equivalent of a lumped inductive and capacitive bandpass filter.
Currently existing evanescent mode waveguides are relatively large in size and weight, especially as the center frequency of operation decreases. This limitation exists since the cross-sectional waveguide dimensions necessary to achieve both the high unloaded quality factor (Q) of resonators and the amount of realizable loading capacitance increases as the filter center frequency decreases. Unloaded Q is inversely proportional to the amount of insertion loss and to the bandwidth of the filter. Therefore, for low loss filters with high selectivity, high unloaded resonator Q is desirable, resulting in the need for a physically large waveguide to maintain performance as the center frequency decreases.
Tuning screws are typically used to form the resonator posts in waveguides. The gaps between the end face of a tuning screw and the wall of the waveguide form shunt capacitances. In air dielectric waveguides, there is a physical limitation to the amount of realizable shunt capacitance that may be achieved, since the physical diameter of the screw must be kept small enough not to perturb the modal performance of the waveguide. By way of example, narrow band filters utilizing tuning screws are expensive to manufacture or difficult to tune because of the necessarily small physical tolerances involved, such as the fineness of the thread of the screw. Another limitation is the allowable physical proximity between a tuning screw's end face and the waveguide wall. It is difficult and expensive to manufacture a tuning screw mechanism that will properly function as a resonator post for a physical proximity that is under one mil (thousandth of an inch), due to the precision required. On the other hand, dielectric filled waveguides, which can increase both unloaded resonator Q and loading capacitance, are not usually employed because it is physically difficult to manufacture and tune them.